Expression of nitric oxide synthase (NOS) was studied in nine human neuroblastoma and two human glioblastoma cell lines. Neuronal NOS (n-NOS) mRNA of approximately 10 kb was detected in four of the nine neuroblastoma cell lines by northern blot analysis using human n-NOS cDNA as a probe. Expression of the n-NOS mRNA was also detected in another neuroblastoma cell line in a subsequent reverse transcriptase polymerase chain reaction (RT-PCR) study, but no n-NOS mRNA expression was observed in the other four neuroblastoma cell lines or in the glioblastoma cell lines. The level of NOS activity correlated well with that of n-NOS mRNA expression in neuroblastoma cell lines expressing n-NOS mRNA. Western blot analysis showed that the n-NOS expressed in neuroblastoma cells was a 160-kDa protein reacted with anti-n-NOS antibody. By using the RT-PCR method, a short n-NOS (n-NOS-2) mRNA with a 315-bp inframe deletion from the entire n-NOS (n-NOS-1) mRNA was detected in the human neuroblastoma cells. The structural diversity of human n-NOS mRNA was demonstrated for the first time.